This invention relates to pressure control for helium cooled superconducting magnet assemblies suitable for magnetic resonance imaging (hereinafter called "MRI"), and more particularly to an improved and simplified helium bath pressure control for systems utilizing a recondenser for recondensing the resultant helium gas back into liquid helium.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing a cryogen such as liquid helium. The extreme cold maintains current flow through the magnet coils after a power source initially connected to the coil (for a relatively short period) is disconnected due to the absence of electrical resistance in the cold magnet coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
The provision of a steady supply of liquid helium to NMRI installations all over the world has proved to be difficult and costly leading to considerable research and development efforts directed at minimizing the need to replenish the boiling liquid helium such as by recondensing the resultant helium gas. Also, it is desirable to avoid the difficulties encountered in storing the necessary reserve supply of liquid helium at cryogenic temperatures of around 4.degree. K (or close to absolute zero) and the related problems of periodically transferring a portion of the liquid helium in the storage reservoir to the liquid helium supply in the MRI superconducting magnet.
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped pressure vessel defining an imaging bore in the central region along its axis. The magnetic field in the imaging bore must be very homogenous and temporally constant for accurate imaging.
Superconducting magnets which recondense the helium gas back to liquid helium are often referred to as zero boiloff (ZBO) magnets. In such ZBO magnets the pressure within the helium vessel must be maintained at pressures above the exterior atmospheric pressure to prevent cryopumping. Cryopumping occurs when helium vessel internal pressure is less than the surrounding atmospheric pressure such that contaminants can be drawn into the helium vessel causing blockages in the magnet penetration which adversely affect MRI performance. Helium vessel pressure below atmospheric pressure can result if the cooling capacity of the cryogenic recondenser exceeds the heat load from the surroundings, namely the cryostat. A typical electrical pressure control system to avoid cryopumping requires a pressure sensor, a controller, wiring, a transducer and a control response which may be either an internal heater which is adjusted by the controller, or a cryocooler speed control system responsive to variations in pressure within the helium vessel.
It has been discovered that operational variations in helium gas pressure within the helium pressure vessel can flex the helium vessel and superconducting magnet coil wires, altering the spatial distribution of current flow through the coils and homogeneity of the magnetic field sufficient to degrade the quality of images produced by the MRI imaging system. The problem is most pronounced in lighter weight pressure vessels being used in lightweight MRI equipment.
Pressure sensor based pressure control systems, whether utilizing a single pressure sensor, or a pair of separated pressure sensors to sense pressure differentials, require an internal pressure sensor exposed to the helium gas within the helium pressure vessel requiring access to the interior of the helium vessel from the exterior pressure control system through a port or tube. This provides a passage through which unwanted heat is introduced into the helium pressure vessel through which resultant Taconis oscillations or pumping action can pump heat into the interior of the helium vessel increasing helium boiling. It is also possible for frost to form in the pressure sensor tubing affecting its operation. Moreover, pressure sensors are more expensive than other types of sensors.
As a result, pressure sensors which respond to variations in pressure within the helium vessel have not proven to be entirely satisfactory in MRI helium pressure control systems.